It is well known in the art to use a downhole pump as a means for artificial lift of oil form a petroleum reservoir, for example to either increase production rates in a naturally producing reservoir or to continue production from a formation at which there is insufficient pressure to naturally produce the fluids to the surface. A downhole pump typically features a pump barrel in which a plunger or piston is slidably disposed. The plunger or piston is attached to the south end of a string of sucker rods that depends into the wellbore to couple the plunger or piston to a suitable pumping unit at the surface that drives reciprocation of the string in order to reciprocate the piston or plunger within the pump barrel.
A standing valve resides at a stationary position at a south end of the pump barrel, while a travelling valve is carried at the south end of the pump piston or plunger for reciprocal movement therewith within the pump barrel under operation of the at-surface pumping unit.
During the upstroke drawing the sucker rod string northward (i.e. in the direction of the wellbore leading toward the pumping unit at the surface), the volume between the rising piston/plunger and the standing valve increases, thereby reducing the pressure inside the pump barrel. With a pressure differential introduced across the standing valve, the higher pressure of the reservoir fluid forces this valve open, thereby introducing the fluid into the interior of the pump barrel. During the upstroke, the hydrostatic pressure of fluid present in the production tubing above the pump barrel keeps the travelling valve closed.
During the subsequent downstroke, the effective internal volume of the pump barrel is decreased by the southward displacement of the piston/plunger, thereby increasing the fluid pressure inside the pump barrel. The pressure differential between the interior and exterior of the pump barrel thus reverses, with the higher pressure fluid inside the pump barrel forcing the standing valve closed, thereby trapping this fluid inside the pump barrel. The rising pressure in the pump barrel increases to a level exceeding the pressure applied to the north side of the travelling valve by the fluid column above the pump barrel, thereby forcing the ball valve of the travelling valve assembly open from the south side thereof and allowing the fluid from this south side of the travelling valve to pass northward therethrough.
It is known in the prior art to add a ball valve lifter to the travelling valve assembly to aid in lifting of the ball valve of the travelling valve assembly from its seat during the downstroke of the downhole pump. Examples of such devices are found in U.S. Pat. No. 7,878,767 and U.S. Patent Application Publication No. 2013/0025846. In these references, a housing is attached to the south end of the travelling valve assembly, and a shaft or piston is slidably disposed in the housing and carries a drag plunger at a south end of the shaft or piston outside the housing. During the downstroke of the downhole pump, the housing moves southward (i.e. further into the wellbore from the surface, or further ‘downhole’) with the travelling valve, but frictional engagement of the drag plunger with the surrounding inner wall surface of the pump barrel resists or prevents movement in the same direction, and/or abutment of the drag plunger against fluid in the pump barrel hydraulically resists or blocks such movement, whereby the device housing moves closer to the drag plunger, thereby relatively displacing the north end of the piston or shaft northward in the housing, until it projects from the housing's north end and knocks the ball valve of the travelling valve from its seated position.
In each of these two prior art devices, the ball lift device is configured to allow the fluid to move northwardly only externally of a shaft or plunger of solid cross-section.
For further reference, additional prior art concerning downhole pumps and associated valve lifters/releasers/assistants includes U.S. Pat. Nos. RE33163, 7,878,767, 4,907,953, 5,628,624, 5,992,452, 5,829,952, 4,867,242, 5,407,333, 7,051,813, 4,708,597, 5,139,398, 5,141,411, 2,344,786, 4,691,735, 5,642,990 4,741,679, 6,481,987, 4,599,054, 4,781,543, 4,781,547 and 5,829,952 and U.S. Patent Application Publications 2013/0025846 and 2005/0053503.
Applicant has developed a number of ball lifter designs that notably depart from the teachings of such prior art solutions in this field.